Negative timezone
## Describe the bug
The load_dataset method do not accept a parquet file with a negative timezone, as it has the following regex:
```
"^(s|ms|us|ns),\s*tz=([a-zA-Z0-9/_+:]*)$"
```
So a valid timestap ```timestamp[us, tz=-03:00]``` returns an error when loading parquet files.

## Steps to reproduce the bug
```python
# Where the timestamp column has a tz of -03:00
datasets = load_dataset('parquet', data_files={'train': train_files, 'validation': validation_files,
                                        'test': test_files}, cache_dir="./cache_teste/")
```

## Expected results
The -03:00 is a valid tz so the regex should accept this without raising an error.

## Actual results
As this regex disaproves a valid tz it raises the following error:
```python
raise ValueError(
                f"{datasets_dtype} is not a validly formatted string representation of a pyarrow timestamp."
                f"Examples include timestamp[us] or timestamp[us, tz=America/New_York]"
                f"See: https://arrow.apache.org/docs/python/generated/pyarrow.timestamp.html#pyarrow.timestamp"
            )
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.11.0
- Platform: Ubuntu 20.04
- Python version: 3.8
- PyArrow version: 5.0.0
